You can also switch their bedding or things that have their sent on it to get them used to the sent. Scarecrow5, have you tried play therapy with them? Play with them both at the same time but separately (need a friend for this) and they learn to associate the other one with positive things, also feed them together, if you can't have them near each other for feeding then do it on opposite sides of a door so they have the sent of the other associated with food. I would try it first and if it doesn't work move on, but only if this is a short term stress issue, if long term get the feli-way. It depends on the cat, yours might be fine or she might react not too good, but that can happen even with Feli-way(I know of cats that get violent when exposed to it.) It's cheaper and it honestly works better because unlike the diffusers it goes with the cat. Also unless you can supervise her a majority of the time when she wears the collar I wouldn't do it. Unless you have a cat you know 100% will not go outside(I don't know about Attitude and Nuts, they mostly don't but once in a blue moon they will so I wouldn't put one on them.) I also wouldn't use it long term. She was slightly stoned but she was still her usual crabby self, just less crabby than usual. This time she was a lot calmer and this was with 2 kittens not just 1. I had already put her back outside but Maude straight up attacked me and left me bleeding. She physically attacked me the first time I brought Attitude in. She also had it on for at most 1 or 2 weeks, it was just enough to let her destress about it. She was afraid to go outside after the dog went out the door and never came back. However Maude would never go outside so she was always in the house with us watching her.
We used one on Maude when Attitude and Nuts first moved in permanently and it helped immensely.